The present invention relates to a process for recovering noble metals from their cyanide complexes in aqueous solution by reaction with red phosphorus.
U.S. Pat. No. 4,392,962 discloses a process for separating seminoble or noble metals from an aqueous solution having compounds of these metals dissolved therein, wherein the aqueous solution is dropped through a column subdivided into two zones. Placed in the upper zone is a layer of granular red phosphorus which may be admixed with active carbon, and placed in the lower zone is a layer of active carbon.
This process can be used for recovering seminoble or noble metals present as ions or in form of complexes. In the latter case, it is however necessary for the complexes to have a low stability, which is determined by the nature of the ligands, i.e. in which the equilibrium of these complexes is substantially on the side of the ionic solution. This condition is met by the silver diamine complex, for example, which can smoothly be reduced to elementary silver in the manner described. Already more difficult to reduce is the silver thiosulfate complex which exists, for example, in spent fixing baths as used for photographic processes. In order to reduce the silver thiosulfate complex to elementary silver, it is necessary for the above known process to be modified, i.e. for it to be effected at increased temperature and at a pH between 8 and 10. But even these flanking operational steps do not permit reducing noble metals which exist as cyanides--these being one of the most important commercial complex systems--to be reduced to elementary metal.
Large quantities of waste water containing 100 mg/l and more silver and/or gold are customarily obtained in the galvanic and gold and silver separating or plating industries. Under commercial aspects, it is not acceptable for such waste water with its high noble metal content to be lost; on the other hand, it is also ecologically pollutive material contaminating surface waters. This is one of the reasons why it is not allowable for the silver content of waste water delivered to a drainage system to exceed a certain upper limiting value. In addition, such waste water is rendered remarkably pollutive by the cyanide it contains. For all these reasons, it is invariably necessary for the waste water, which is to be delivered to a drainage system, to be first subjected to an expensive multistage clarification and decontamination process during which the noble metals are recovered by the exchange of ions, e.g. by a process as disclosed in Japanese Specification JP-AS No. 55-25915.
It is therefore desirable to have a process for recovering noble metals existing as cyanide complexes in aqueous solution which is easy to carry out under commercially acceptable conditions and permits the cyanides to be simultaneously destroyed.